The existence of the unmyelinated primary afferent fibers in the dorsal funiculus, which is known as a large myelinated primary afferent fiber tract, has been clearly demonstrated in the second sacral segment of cat and rat spinal cords. The immediate question is whether these fibers ascend in the dorsal and dorsolateral funiculi or whether they remain in the segment of origin. This is assessed by comparing numbers of unmyelinated fibers on the operated side with those on the normal side in the spinal segment just cranial to the dorsal rhizotomies and in the C2 segment. Adult domestic cats weighing 2-5 kg are used in this study. Cats are anesthetized with 40 mg/kg sodium pentobarbital and multiple dorsal rhizotomies are performed on one side of the spinal cord. The animals are sacrificed one week after the operation and the appropriate spinal cord segments are processed for electron microscopy. This survival time is adequate for the death of axons to become apparent but before any possible sprouting takes place. The numbers of normal myelinated fibers and degenerating myelinated fibers are counted from the montages of the dorsal funiculus and dorsolateral funiculus. The difference in numbers between the normal and the deafferented sides will represent the primary afferent fibers that ascend in these regions of the spinal cord.